A novel primary stability test method for artificial acetabular shells considering vertical load during level walking and shell position

Uncemented acetabular shell primary stability is essential for optimal clinical outcomes. Push-out testing, rotation testing, and lever-out testing are major evaluation methods of primary stability between the shell and bone. However, these test methods do not consider shell loads during daily activity and shell installation angle. This study proposes a novel evaluation method of acetabular shell primary stability considering load during level walking and acetabular installation angles such as inclination and anteversion. To achieve this, a novel primary stability test apparatus was designed with a shell position of 40° acetabular inclination and 20° anteversion. The vertical load, corresponding to walking load, was set to 3 kN according to ISO 14242–1, which is the wear test standard for artificial hip joints. The vertical load was applied by an air cylinder controlled by a pressure-type electro-pneumatic proportional valve, with the vertical load value monitored by a load cell. Torque was measured when angular displacement was applied in the direction of extension during the application of vertical load. For comparison, we also measured torque using the traditional lever-out test. The novel primary stability test yielded significantly higher primary stabilities; 5.4 times greater than the lever-out test results. The novel primary stability test failure mode was more similar to the clinical failure than the traditional lever-out test. It is suggested that this novel primary stability test method, applying physiological walking loads and extension motions to the acetabular shell, better reflects in vivo primary stability than the traditional lever-out test.


Introduction
Aseptic loosening is the most common reason for revision surgery of uncemented acetabular shells in total hip arthroplasties (THAs) commonly used in clinical practices [1][2][3][4].Early osseointegration is required to prevent aseptic loosening, and for this purpose, adequate primary stability to reduce micromotion at the bone-implant interface is key [5][6][7][8].
Uncemented acetabular shells are fixed by the press-fit technique, in which an oversized or geometrically-matched shell is inserted into reamed host bone by impact loads applied using a surgical hammer through an inserter.The impact load caused by manual hammer blows can range widely, from 5 to 27 kN [9][10][11].
Conventional uncemented acetabular shells are clinically successful in primary THAs [12][13][14].However, concerns exist that the clinical results for new products may be inferior to those of conventional products [15][16][17][18].Furthermore, to improve dislocation resistance and range of motion, the use of large-diameter femoral heads has recently increased [19], raising concerns of aseptic loosening due to increased frictional torque between the femoral head and liner [20].Based on this, it is important to evaluate the testing methods of primary stability between the shell and bone to meet essential requirements for new product development.
The push-out test, rotation test, and lever-out test are major methods for primary stability evaluation between the shell and bone [21][22][23][24].However, these methods do not consider shell load during daily activities and shell installation angle.The vertical load associated with level walking is at a maximum at heel strike and toe off during the extension phase, with the artificial joint friction torque also reaching a maximum at this stage [25,26].The lever-out test is also reported for evaluating primary stability in the direction of flexion-extension and abduction-adduction [23,27].However, the relationship between pelvis geometry, vertical load, and direction of motion, such as flexion-extension associated with shell installation angle, is inadequate for the lever-out test.With this in mind, we considered it necessary to simulate installation angle, in vivo load, and acetabular shell anatomical motion direction to effectively evaluate primary stability between bone and shell.
Firstly, we propose a novel primary stability evaluation method for the acetabular shell that considers load during level walking and installation angles, such as acetabular inclination and anteversion.Secondly, primary stability evaluations are compared between the novel primary stability test and the conventional lever-out test.

Acetabular components
The traditional lever-out test and the novel primary stability test were performed with the uncemented type acetabular shell (Continuum Acetabular Shell, Ref# 00-8757-050-01, Zimmer Biomet G.K.).This shell has a 50 mm outer diameter and a highly porous tantalum 3D surface.
As a standardized replacement for human pelvis cancellous bone, blocks of cellular rigid polyurethane foam were used with a density of 15 pcf and a Young's modulus of 68.4 MPa (Sawbones; Pacific Research Laboratories Inc., Vashon, WA, USA.Product number: #1522-1300).These properties reflect those of human acetabular trabecular bone (Young's modulus: 116.4 ± 86.7 MPa) [28].The polyurethane foam block was cut into �90 mm × 90 mm × t40 mm, then the center portion of the polyurethane foam block reamed using a 49 mm surgical reamer (1 mm under reaming) with a custom-made instrument mounted to a bench drill.

Lever-out test
A photograph of the fixation method for the lever-out test is shown in Fig 1 .The acetabular shell was installed into the reamed acetabular bone model with a uniaxial universal testing machine (Autograph AGS-X, Shimadzu Corporation).The applied force was 5 kN [9] in displacement-controlled mode at a rate of 5 mm/min [22].Following fixation, a custom rigid stainless-steel fixator was threaded into the inserter attachment hole of the acetabular cup.The lever-out test was then conducted by pulling the custom rigid fixator with a chain at a rate of 5˚/min until the shell detached from the acetabular bone model (Fig 2) [23].The lever-out torque was calculated by multiplying the load and the moment arm [23].

Novel primary stability test
A photograph of the novel primary stability test apparatus is shown in Fig 3 .The jig for fixturing the acetabular bone model was prepared to simulate an acetabular inclination and anteversion of 40˚and 20˚, respectively [29,30].The jig for fixturing the acetabular shell was designed such that the shell center and shaft center of rotation coincided with an acetabular inclination of 40˚and anteversion of 20˚.The jig for fixturing the acetabular shell was fixed using low melting point alloy (U-60, Osaka Asahi Co., Ltd.) to the acetabular shell.The acetabular shell was then installed into the reamed acetabular bone model with a uniaxial universal testing machine (Fig 4).The applied force was 5 kN [9] in displacement-controlled mode at a rate of 5 mm/min [22].The acetabular shell with the bone model was set in the jig for fixturing the acetabular bone mode, then the jig for fixturing the acetabular shell was attached to the shaft.The vertical load, corresponding to walking load, was set to the wear test standard for artificial hip joints of 3 kN, according to ISO 14242-1.The vertical load was applied by an air cylinder controlled by a pressure-type electro-pneumatic proportional valve (VEP3121-1, SMC Corporation), and vertical load value monitored by the load cell (LMR-S-5KNSA2, Kyowa Electronic instruments Co., Ltd.).In the novel primary stability test, torque was applied, under vertical load, to the sprocket an angular displacement of 5˚/min in the direction of extension by raising the chain using a universal testing machine (Autograph AGS-X, Shimadzu Corporation).Data recording and vertical load commands were carried out using a Labview interface (National instruments, Austin, TX, USA) with a rate of 100 Hz.Torque was calculated by multiplying the load on the pull-up chain and the moment arm (sprocket pitch diameter).

Polar gap measurement
Following fixation with a 5 kN load, the acetabular shell position was measured in relation to the synthetic bone.A schematic diagram of the polar gap measurement is shown in Fig 5 .The polar gap is calculated geometrically from the acetabular shell dimensions, reaming radius, and protruding shell height by the following equation: Where "Δ" is the polar gap, "H m " is the protruding shell height, "h r " is the acetabular shell rim height, "R s " is the hemispherical acetabular shell outer radius or hemielliptical acetabular shell minor axis, and "r r " is the reaming radius.The protruding shell height "H m " was measured at four points separated by 90 degrees, using a height gauge (HDS-H30C, Mitutoyo Corporation), and the averaged value used in Eq (1).

Statistical analyses
Differences between the peak torques of the lever-out test and the novel primary stability test were assessed using the t-test (α = 0.05).The sample size was 5 for each test type.Statistical analyses were conducted using appropriate software (JMP Pro 16, SAS Institute Inc.).

Polar gap measurement
The polar gap results of the two test methods are listed in Table 1.A negative polar gap value indicates that the acetabular shell migrated to the synthetic bone.All tested shells were confirmed to be completely seated in the 1-mm under reamed condition.

Primary stability
Representative torque-angle curves for the lever-out test and novel primary stability test are shown in Figs 6 and 7, respectively.In the lever-out test, as angular displacement was applied,  the torque increased, reaching a maximum value, then decreasing (Fig 6).In the novel primary stability test, torque increased with angular displacement to plateau, then underwent some fluctuation (Fig 7).Furthermore, in all novel primary stability test experiments, the vertical load was controlled within 3 kN ± 1%.From these torque-angle curves, the lever-out test primary stability was defined as the maximum torque (Peak Torque in Fig 6), and the primary stability of the novel primary stability test was defined as the first peak torque in the plateau (Peak Torque in

Discussion
The primary stability of uncemented acetabular shells is essential for effective biological fixation.This study compared the novel primary stability test method for acetabular shells that simulates walking loads, to the traditional lever-out test, and examined method validity.
The acetabular shell failure mode after the novel primary stability testing was loss of fixation with maintenance of center of rotation in the reamed section of the trabecular bone model ( Fig 10).The acetabular shell failure mode after lever-out testing involved the upper end of the shell acting as a fulcrum, while the lower end of the shell was pulled out from the trabecular bone model (Fig 9).Artificial hip joint acetabular shells are loaded by sliding torque between the femoral head and liner.As the bearing surface between the femoral head and liner is convex and concave, the friction torque is generated at the center of the sphere.Furthermore, the hip joint articular surface is always subjected to compressive force due to the surrounding muscle tissue and walking load.Clinically, rotated acetabular shells and liners are confirmed to dislodge in the acetabulum [31,32].Based on these reports, it is considered appropriate to evaluate the primary stability between the acetabular shell and the bone during walking load by evaluating resistance to rotational torque at the center of the sphere.Therefore, we consider that the primary stability results obtained using the proposed novel primary stability test are more appropriate in reflecting in vivo stability than the traditional lever-out test.Comparing the peak torques from the lever-out test and novel primary stability test, the novel primary stability test peak torque was 61.5 ± 3.5 Nm, and the peak torque by the leverout test was 11.4 ± 1.0 Nm (Fig 8).Thus, primary stability by the novel primary stability test was 5.4 times greater than that obtained using the lever-out test, a significant difference.The primary stability in this study shows the shell-bone interface static strength.In general, cyclic fatigue strength is lower than static strength, and Ti-6Al-4V, a widely used implant material, is reported to reduce fatigue strength to about half [33].Furthermore, when evaluating industrial products, minimizing individual differences in products and fluctuations in the usage environment is a focus to ensure safety.IEC 60601-1, an international standard for the safety of electrical medical equipment, indicates that a safety factor of 6 for mechanical strength is required.Considering these, the in vivo torque for which the acetabular shell is stably fixed to the bone for a long period of time is estimated as below 0.9 Nm by the lever-out test, while the novel primary stability test indicates 5.1 Nm.The maximum torque is reported between 1.0 and 3.9 Nm in vivo for the extension direction during level walking using an artificial hip joint incorporating a 6-axis force sensor [25,26].The primary stability estimated from the lever-out test results indicates a requirement for additional acetabular shell fixation with supplemental screws.However, it is reported that with good bone condition, there is no loosening and good clinical outcomes are obtainable without supplemental screws [34].The injury risk to neurovascular structures increases with the number of screws used [35].Furthermore, the use of supplemental bone screws allows partial bone ingrowth around the screw holes, however, does not allow for ingrowth over the area of the screw hole [36].These findings suggest that when comparing the load generated in vivo and initial fixation force, the initial fixation force obtained in the lever-out test may underestimate the implant, resulting in unnecessary prosthetic alterations.
There are some limitations to this study.First, primary stability was only evaluated in the extension direction for the peak vertical loading condition of level walking in this study, other biomechanical test conditions were not evaluated.Second, the acetabular bone model used has uniform material properties, however, the properties of natural bone are heterogeneous, the impact of which requires further examination.Third, the acetabulum was assumed hemispherical and defect-free.However, the natural acetabulum may have defects, including the acetabular notch, lateral defects, and an acetabular fossa, the influence of which also requires examination [37][38][39][40].Fourth, despite the simulation of shell installation position, in vivo loads are dynamic, thus individual differences such as spino-pelvic dynamics, supine versus standing as well as limb positions were not reflected in this study.Fifth, effective biological fixation is necessary to obtain clinically good long-term fixation, and is influenced by factors such as primary stability, biocompatibility, and surface roughness.Given these, we believe that the novel test method presented is more appropriate than traditional methods to understand the relationship between acetabular shell geometric design and primary stability.However, it is difficult to directly translate these results to clinical practice, and comprehensive evaluation considering the complex biological environment is necessary.

Conclusion
The novel primary stability test method developed in this research applies physiological walking loads and extension motions to the acetabular shell.This testing method better reflects primary stability in vivo than the traditional lever-out test.

Fig 5 .
Fig 5. Schematic diagram of the polar gap measurement.https://doi.org/10.1371/journal.pone.0296919.g005 Fig 7).A comparison of the peak torque between the lever-out test and the novel primary stability test is shown in Fig 8.The primary stability by the novel primary stability test was 5.4 times greater than that by the lever-out test, a significant difference.Photographs of the representative post-test specimens from the lever-out test and the novel primary stability test are shown in Figs 9 and 10.In the lever-out test, the upper end of the shell acted as a fulcrum, with the lower end pulling out of the acetabular bone model (Fig9).In the novel primary stability test, the acetabular shell rotated while maintaining the center of rotation in the reamed recess (Fig10).